This invention pertains to voltage multiplying devices which utilize the vector inversion principle and in particular to spiral generators.
A spiral generator is basically a pulse generator in which a number of electric vectors are transiently rearranged to be in series addition. The spiral generators which were first reported by Fitch and Howell (PROC. IEEE, Volume 111, No. 4, pages 849-855, 1964), are pulsed high voltage generators that provide a multiplication of the initial charging voltage. Briefly, a spiral generator is a rolled-up strip transmission line (stripline) with an additional layer of dielectric as an insulation between turns. Each of the N turns of the spiral consists both of an active and a passive stripline. The outer end of the active line is connected to a switch. The remaining ends of both lines are open. The pulsed high voltage output terminals are DC connected, with one terminal, normally the reference terminal, being connected to the outer active stripline and the other terminal being connected to the inner passive stripline. Initially, some voltage of V.sub.c is applied to the outer end of the active stripline causing both striplines to be charged to the voltage V.sub.c with the electric field vectors of the active lines oriented in one direction and the electric field vectors of the passive lines oriented in the opposite direction.
The generation of the high voltage output pulse is caused by the alignment of the active line vectors with the passive line vectors. The alignment is caused by the closing of a switch which is normally connected to the outer end of the active line. When the switch closes, a wave travels down the active line, reflects off the open circuit at the inner end, and travels back to the switch, reversing the polarity of the active line relative to it's initial charge. The voltage across each active and passive line pair is then ideally -2V.sub.c. If there are N pairs of active/passive lines then the total output voltage ideally is given by the following equation: EQU V.sub.o =-2NV.sub.c
The time for the spiral generator to erect is given by the following equation: EQU .tau.=2.pi.ND/v
where "D" is the mean diameter of the spiral and "v" is the velocity of light in the dielectric.
After the wave reaches the switch again, it reflects off the short circuit of the closed switch with the opposite sign and begins to travel back down the active line. As it proceeds it de-erects the generator, which returns to zero output after a total elapse time of 2.tau.. After this voltage continues to oscillate up and down with a period equal to 2.tau..
In high voltage applications, spiral generators presented heretofore have a set of problems which are addressed and corrected by the invention presented herein. First of all, there is significant mechanical stress due to the electric field between the striplines when the unit is charged to its initial value, typically in the thousands of volts range. Secondly, there are two flash-over problems, the flash-over which occurs around the edge of the dielectric strips and the flash-over caused by corona from the edges of the conducting tape. Furthermore, there is a punch-through problem connected with the innermost conducting layer where it passes over the end of the dielectric tape. The punch-through of course breaks down the insulation properties of the dielectric and provides a path for electrical conduction and therefore reduction of the output voltage of the generator. In addition to the above, the switch which starts the process of high voltage pulse generation has a certain amount of inductance. The inductance, of course, has a significant effect upon the rise time of the switch. The rise time of the current through the switch has a direct effect upon the output voltage of a generator.
The invention presented herein significantly reduces or eliminates the above described problems. Other advantages and attributes of this invention will be apparent upon reading of the text hereinafter.